How many did Chernobyl kill? More than 4,000….

April 26 will mark the 25th anniversary of the Chernobyl nuclear accident. I’ll be publishing a story on the day that, with the help of TIME’s Kiev-based stringer James Marson, will show how the effects of the meltdown continue to be felt in the region. Nuclear accidents require the work of generations to clean-up. That’s a troubling lesson for Japan.

One piece of information both James and I have found almost impossible to ascertain is the number of people actually killed by the release of radiation during the disaster. The authoritative figure often cited comes from a UN’s World Health Organization’s press release dated Sept. 5, 2005 that states that “A total of up to 4000 people could eventually die of radiation exposure from the Chernobyl nuclear power plant (NPP) accident nearly 20 years ago.”

Turns out, however, that this figure was never meant to be the definitive estimate. WHO’s spokesman Gregory Härtl says it’s a partial figure selected by the public relations company that put together the press release; it only refers to deaths in the most heavily affected regions near the plant.

But even that estimate may be low. After all, radiation spread beyond Europe in 1986. Other areas within the northern hemisphere—including Asia, Africa and the Americas—were also contaminated by the accident. Taking in this consideration, the Union of Concerned Scientists (UCS) put the global death toll closer to 27,000 rather than 16,000.

Why such discrepancies? For one, no accurate dose estimates on the affected population were made until after the break-up of the Soviet Union. As a result, Belarus, Russia, and Ukraine all use different dosimetry techniques and dose definitions. What’s more, the period after the Soviet Union saw plummeting life expectency and poor health due to alcohol and tobacco consumption and poverty–that makes measuring “excess death” a highly inexact exercise. And then, of course, is the uncertainty of the effects of low-dose radiation itself, which I’ve written about here. No one knows for sure how many cancers low-dose radiation causes, and given that 20 percent of deaths come from cancer anyway, it’s almost impossible to ascertain which environmental factors (smoking, industrial pollution, radiation, etc) caused the disease.

So what figure will James and I use in our piece? I think the sensible conclusion is the one drawn by the Union of Concerned Scientists: “Because of these inherent uncertainties, it is perhaps most appropriate to only cite order-of-magnitude results: the numbers of excess cancer deaths worldwide will be in the tens of thousands.”